1. Field
The present invention relates generally to calibration of sensors, and more particularly, to a method and apparatus for calibration of sensor signals.
2. Background
Currently there are many different types of sensors. Often times, the sensors have output signals that need to be adjusted or “conditioned” to remove any anomalous or otherwise unwanted effects. For uniformity of application of a particular design, it is desirable that the output signals for these sensors are consistent from sensor to sensor. However, differences introduced by the manufacturing of the sensors will cause variation in the operating response of the sensors. Further, external environmental influences such as temperature will also affect the operating response.
Typically, sensor transducer circuits are used to convert the output signals into linearized and temperature-compensated signals. These linearized and temperature-compensated signals are then calibrated with respect to a certain null level by a calibration process known as an offset or zero calibration process. The output from the offset calibration process is then calibrated against a certain full-scale value by a calibration process referred to as a span or gain calibration process.
Signal conditioning is typically performed either by digital signal processors or by analog signal processors. When the signal conditioning is performed by a digital signal processor, sensor output signals are first converted into digital signals, which are digital representations of the analog format of the sensor output signals. Then, the digital signal processor processes the digital signals using one or more calibration coefficients. The calibration coefficients are stored in digital memory. The corrected digital signals may then be used as input signals for a digital system (e.g. a microprocessor).
Analog signal processors can be split into two main categories—those having digital storage of calibration coefficients and those without. Analog signal processors without digital storage of calibration coefficients calibrate sensor signals through the use of variable resistors such as potentiometers or laser trim resistors. In laser trimming, a laser is used to ablate the resistor material from the substrate, raising its resistor value.
Analog signal processors with digital storage of calibration coefficients include Digital to Analog Converters (DAC's), and can be seen as digital controlled potentiometers. DAC's typically consist of a network of resistors that are digitally routed in such a way to adjust the total resistance the signal will see. Thus, generally, DAC's are used as variable resistors to calibrate the signal.
Sensors tend to have proportionally less and less output gain as pressure is increased. In addition, all sensors have some shift in zero in addition to non-repeatability when subjected to temperature variations. Further, under certain conditions, the sensor output does not change linearly with temperature and a second-order correction term must be incorporated. The result is that sensors have both a linear and a second-order error. Various current sensor transducer circuits provide separate adjustments for signal offset (i.e., a deviation from an expected signal output), span (i.e., the span of the signal output), linearization (i.e., the linearity of the signal output), temperature coefficient (TC)-offset (i.e., the offset based on the TC) and TC-span (i.e., the span based on the TC). Although these circuit arrangements can deal with a wide range of sensor signals, they are typically complex. The complexity of these circuit arrangements increase both design and manufacturing costs.
It is desirable to address the deficiencies of existing approaches to sensor signal compensation in a lower cost and simpler manner.